1. FIELD OF THE INVENTION
The present invention relates to a negative feedback amplifier and more particularly concerns an amplifier having a variable gain (hereinafter referred to as the variable gain amplifier) which is suited to be implemented in the form of an integrated circuit.
2. DESCRIPTION OF THE PRIOR ART
As the variable gain amplifier which is capable of amplifying a signal voltage with a given amplification factor with high accuracy, there has been hitherto known a negative feedback amplifier which comprises an amplifier provided with a feedback loop including high precision operational resistors. In order to allow the gain of the negative feedback amplifier to be varied in a wide range, a voltage divider circuit composed of a number of high precision resistors has to be provided in the feedback path, involving a complicated and expensive circuit configuration. Further, difficulty is encountered in the attempt to implement the negative feedback amplifier in the form of an integrated circuit. Namely, when the resistors are realized in the integrated circuit, the resistance value of the integrated resistors becomes unstable under the influence of voltage, making it difficult to attain a high precision resistance. Under these circumstances, realization of the negative feedback amplifier which requires high precision resistors has been difficult.
As an attempt to overcome the difficulties described above, there has been proposed a variable gain amplifier, the gain of which is controllable by controlling the dividing ratio of voltage in accordance with the duty cycle or duty ratio of a control pulse signal. A typical example of a variable gain amplifier which is operative on the basis of such a principle is shown in FIG. 1. An input signal to be amplified is supplied from an input signal source 50 to a CR-smoothing circuit composed of a resistor 111 and a capacitor 130 through a switch 150, wherein the output from the smoothing circuit is supplied to an operational amplifier 200. By controlling the on-off operation of the switch 150, it is possible to derive a smoothed d.c. output signal through the buffer amplifier 200 having a high input impedance, which signal is in proportion to the duty cycle .alpha. of the pulse voltage signal applied to the smoothing circuit. It will be appreciated that the voltage divider circuit comprising a number of resistors is replaced by the combination of the switch 150 and the smoothing circuit 111, 130.
When the output voltage of the signal source 50 is represented by V.sub.i, the output voltage of the amplifier is represented by V.sub.o and the resistance value of the smoothing circuit is represented by R, then the output voltage V.sub.o is determined in the following manner on the assumption that the switch is an ideal one. EQU (V.sub.i -V.sub.o).alpha.T.sub.o .multidot.1/R=V.sub.o (1-.alpha.)T.sub.o .multidot.1/R, Therefore, V.sub.o =V.sub.i .alpha. (1)
where T.sub.o represents the period of the pulse voltage signal. It is apparent that the output voltage V.sub.o is determined with high accuracy from the applied voltage V.sub.i and the duty cycle or duty factor .alpha.. In this manner, the circuit which comprises the resistor 111 and the capacitor 130 and serves for voltage division in dependence on the duty factor of the on-off switch 150 makes it possible to control the gain of the amplifier 200 with an extremely high accuracy. However, the variable gain amplifier circuit of the type described above is disadvantageous in that a high speed response can not be attained. For example, in order to attain a gain of the amplifier with precision in the order of 0.1% by reducing ripple components contained in the output signal from the smoothing circuit, the time constant .tau. of the CR-smoothing circuit must be approximately equal to 500 T.sub.o. Further, the time constant .tau. must be approximately equal to 3500 T.sub.o when a time 7.tau. is required for settling the output voltage responsive to a steeply changing input voltage. Thus, the response of the amplifier is made very slow with respect to the input signal. To avoid such inconveniences, it has also been proposed to adopt a sample and hold technique to effect the feedback control at a sampling period aproximately equal to the time constant of the CR-smoothing circuit, thereby to attain the rapid response by settling the input signal for every several samplings. However, unless the CR-time constant of the smoothing circuit coincides with the sampling period, the number of the settling cycles as required will be increased. Further, a problem lies in respect of the amplification and the complicated circuit configuration.